Circadian timing systems are affected by aging. However the mechanism by which aging affects these systems has not been described. Specifically, how the control of peripheral oscillators, and thereby circadian organization of organ function is affected by aging has not been investigated. By using in vivo electrophysiological recording, new techniques in dynamic recording of gene expression in vitro, and neuronal transplants, I intend to investigate mechanisms by which the circadian system of the rat is affected in advanced age, and how these changes manifest themselves in behavior and physiology, and in the rats capacity to adapt to changes in the temporal environment. [unreadable] Specific Aim 1. I will test the hypothesis that aging leads to a breakdown of the temporal control of physiology and peripheral oscillators by the SCN. [unreadable] Specific Aim 2. I will test the hypothesis that the locus of age-related changes in steady-state temporal organization and phase resetting is the SCN. [unreadable] In addition to gaining a better understanding of how the central circadian pacemaker in the brain controls behavior and physiology, these experiments should help us to better understand the aging components of such issues as the health problems common in chronic shift workers and the role of circadian rhythms in acute cardiovascular disease. [unreadable] [unreadable]